Multifunctional Mechanical Metamaterials with Embedded Triboelectric Nanogenerators

The flexibility of planar triboelectric nanogenerators (TENGs) enables them to be embedded into structures with complex geometries and to conform with any deformation of these structures. In return, the embedded TENGs function as either strain‐sensitive active sensors or energy harvesters while negligibly affecting the structure's original mechanical properties. This advantage inspires a new class of multifunctional materials where compliant TENGs are distributed into local operational units of mechanical metamaterial, dubbed TENG‐embedded mechanical metamaterials. This new class of metamaterial inherits the advantages of a traditional mechanical metamaterial, in that the deformation of the internal topology of material enables unusual mechanical properties. The concept is illustrated with experimental investigations and finite element simulations of prototypes based on two exemplar metamaterial geometries where functions of self‐powered sensing, energy harvesting, as well as the designated mechanical behavior are investigated. This work provides a new framework in producing multifunctional triboelectric devices.     

A Nonmetallic Stretchable Nylon‐Modified High Performance Triboelectric Nanogenerator for Energy Harvesting

As a new energy harvesting strategy, triboelectric nanogenerators which have a broad application prospect in collecting environmental energy, human body mechanical energy, and supplying power for low‐power electronic devices, have attracted extensive attention. However, technology challenges still exist in the stretchability for the preparation of some high‐performance triboelectric materials. In this work, a new strategy for nonmetallic nylon‐modified triboelectric nanogenerators (NM‐TENGs) is reported. Nylon is introduced as a high performance friction material to enhance the output performance of the stretchable TENG. The uniform matrix reduces the difficulty of heterogeneous integration and enhances the structural strength. The open‐circuit voltage (V_OC) and short‐circuit current (I_SC) of NM‐TENG can reach up to 1.17 kV and 138 µA, respectively. The instantaneous power density reaches 11.2 W m^?2 and the rectified output can directly light ≈480 LEDs. The transferred charge density is ≈100 µC m^?2 in one cycle when charging the capacitor. In addition, a low‐power electronic clock can be driven directly by the rectified signal without additional circuits. NM‐TENG also has high enough strain rate and can be attached to the human body for energy harvesting effectively. This work provides a new idea for fabrication of stretchable TENGs and demonstrates their potential application.     

Triboelectric and Piezoelectric Effects in a Combined Tribo‐Piezoelectric Nanogenerator Based on an Interfacial ZnO Nanostructure

In this contribution, combined triboelectric and piezoelectric generators (TPEG) with a sandwich structure of aluminum‐polydimethylsiloxane/polyvinylidene fluoride composite‐carbon (Al‐PPCF‐Carbon) are fabricated for the purpose of mechanical energy harvesting. Improved by the surface modification of PPCF with zinc oxide (ZnO) nanorods through a hydrothermal method, the TPEG generates an open‐circuit voltage (V_oc) of ≈40 V, a short‐circuit current (I_sc) of 0.28 μA with maximum power density of ≈70 mWm^?2_, and maximum conversion efficiency of 34.56%. Subsequently, in order to understand the transduction mechanism of the triboelectric and piezoelectric effects, analyses focusing on the potential composition ratio in the final output and the impact of ZnO interfacial nanostructure are carried out. The observed potential ratio between triboelectric and piezoelectric effects is 12.75:1 and the highest potential improvement by ZnO nanorods of 21.8 V is achieved by the TPEG fabricated with spacer. Finally, the relationships between the voltage, power density, conversion efficiency, and the external load resistances are also discussed. Overall, the fabricated TPEG is proved to be a simple and effective nanogenerator in mechanical energy conversion with enhanced output potential and conversion efficiency.     

Direct‐Current Triboelectric Generator

The first direct‐current triboelectric generator (DC‐TEG) based on sliding electrification for harvesting mechanical energy from rotational motion is reported. The DC‐TEG consists of two rotating wheels and one belt for connecting them, which are made of distinctly different triboelectric materials with a specific requirement. During the rotation, the contact‐induced electrification and the relative sliding between the two wheels and the belt can induce a continuous increase of the accumulated positive and negative triboelectric charges at the two rotating wheels, respectively, resulting in a Corona discharge and producing the observed current through an external load. The DC‐TEG can deliver an open‐circuit voltage of larger than 3200 V and a maximum power of 100 μW under an external load of 60 MΩ at a rotational speed of 1000 r min^–1. By designing a point metal discharge electrode near the accumulated positive charges on the metal wheel, the instantaneous short‐circuit current can be up to 0.37 mA. The DC‐TEG can be utilized as a direct power source to light up 1020 serially connected commercial light‐emitting diodes (LEDs) and the produced energy can also be stored in a capacitor for other uses. This work presents a DC‐TEG technology to harvest mechanical energy from rotational motion for self‐powered electronics.     

Sustainable Nanotextured Wave Energy Harvester Based on Ferroelectric Fatigue‐Free and Flexoelectricity‐Enhanced Piezoelectric P(VDF‐TrFE) Nanofibers with BaSrTiO3 Nanoparticles

Here, ultrathin, flexible, and sustainable nanofiber‐based piezoelectric nanogenerators (NF‐PENGs) are fabricated and applied as wave energy harvesters. The NF‐PENGs are composed of poly(vinylidene fluoride‐co‐trifluoroethylene) (P(VDF‐TrFE)) nanofibers with embedded barium strontium titanate (BaSrTiO₃) nanoparticles, which are fabricated by using facile, scalable, and cost‐effective fiber‐forming methods, including electrospinning and solution blowing. The inclusion of ferroelectric BaSrTiO₃ nanoparticles inside the electrospun P(VDF‐TrFE) nanofibers enhances the sustainability of the NF‐PENGs and results in unique flexoelectricity‐enhanced piezoelectric nanofibers. Not only do these NF‐PENGs yield a superior performance compared to the previously reported NF‐PENGs, but they also exhibit an outstanding durability in terms of mechanical properties and cyclability. Furthermore, a new theoretical estimate of the energy harvesting efficiency from the water waves is introduced here, which can also be employed in future studies associated with various nanogenerators, including PENGs and triboelectric nanogenerators.     

Elastic-Connection and Soft-Contact Triboelectric Nanogenerator with Superior Durability and Efficiency

Triboelectric nanogenerator (TENG) has received tremendous attention in ambient energy harvesting, especially for ocean wave energy. However, the technology is generally challenged to obtain excellent durability and high efficiency simultaneously, which primarily overshadows their further industrial-scale applications. Here, a dual-mode and frequency multiplied TENG with ultrahigh durability and efficiency for ultralow frequency mechanical energy harvesting via the elastic connection and soft contact design is proposed. By introducing the spring and flexible dielectric fluff to the novel pendulum-like structural design, the surface triboelectric charges of TENG are replenished in soft contact mode under the intermittent mechanical excitation, while the robustness and durability are enhanced in non-contact working mode. The fabricated TENG results in a continuous electrical output for 65 s by one stimulus with a high energy conversion efficiency, as well as negligible change of output performance after a total of 2 000 000 cycles. Moreover, integrated with the power management circuit, the TENG array is demonstrated to drive the electronics by effectively harvesting wind and water wave energy as a sustainable energy source. This work paves a new pathway to enhance the robustness, durability, and efficiency of the TENG that resolves the bottleneck of its practical applications and industrialization.     

Tilting‐Sensitive Triboelectric Nanogenerators for Energy Harvesting from Unstable/Fluctuating Surfaces

The invention of triboelectric nanogenerators provides an opportunity to utilize previously wasted mechanical energy. The sway energy of ships that affects navigation and comfort on board has been considered negative in the past. Here, a tilting‐sensitive triboelectric nanogenerator (TS‐TENG) that can effectively harvest energy from unstable/fluctuating surfaces is demonstrated by using the sway energy of ships. The device adopts integrated blade structures on sliders, which make it sensitive to tilts and guarantee its power output. The response of the device to tilt agitations of different slopes and frequencies is systematically investigated. Rotational symmetry configuration is used to improve the motion stability of the device by excluding extra torque on the sliders. The peak power density and average power density of the TS‐TENG can reach 1.41 and 0.1 W m^?3, respectively, in low‐frequency and low‐amplitude fluctuating conditions. By the excellent performance of harvesting energy from unstable/fluctuating surfaces, the TS‐TENG is considered promising for powering various distributed sensor devices on the ship for smart ships.     

Magneto-Mechanical Metamaterials with Widely Tunable Mechanical Properties and Acoustic Bandgaps

Mechanical metamaterials are architected manmade materials that allow for unique behaviors not observed in nature, making them promising candidates for a wide range of applications. Existing metamaterials lack tunability as their properties can only be changed to a limited extent after the fabrication. Herein, a new magneto-mechanical metamaterial is presented that allows great tunability through a novel concept of deformation mode branching. The architecture of this new metamaterial employs an asymmetric joint design using hard-magnetic soft active materials that permits two distinct actuation modes (bending and folding) under opposite-direction magnetic fields. The subsequent application of mechanical compression leads to the deformation mode branching where the metamaterial architecture transforms into two distinctly different shapes, which exhibit very different deformations and enable great tunability in properties such as mechanical stiffness and acoustic bandgaps. Furthermore, this metamaterial design can be incorporated with magnetic shape memory polymers with global stiffness tunability, which also allows for the global shift of the acoustic behaviors. The combination of magnetic and mechanical actuations, as well as shape memory effects, impart wide tunable properties to a new paradigm of metamaterials.     

Cellulose II Aerogel‐Based Triboelectric Nanogenerator

Cellulose‐based triboelectric nanogenerators (TENGs) have gained increasing attention. In this study, a novel method is demonstrated to synthesize cellulose‐based aerogels and such aerogels are used to fabricate TENGs that can serve as mechanical energy harvesters and self‐powered sensors. The cellulose II aerogel is fabricated via a dissolution–regeneration process in a green inorganic molten salt hydrate solvent (lithium bromide trihydrate), where. The as‐fabricated cellulose II aerogel exhibits an interconnected open‐pore 3D network structure, higher degree of flexibility, high porosity, and a high surface area of 221.3 m² g^?1. Given its architectural merits, the cellulose II aerogel‐based TENG presents an excellent mechanical response sensitivity and high electrical output performance. By blending with other natural polysaccharides, i.e., chitosan and alginic acid, electron‐donating and electron‐withdrawing groups are introduced into the composite cellulose II aerogels, which significantly improves the triboelectric performance of the TENG. The cellulose II aerogel‐based TENG is demonstrated to light up light‐emitting diodes, charge commercial capacitors, power a calculator, and monitor human motions. This study demonstrates the facile fabrication of cellulose II aerogel and its application in TENG, which leads to a high‐performance and eco‐friendly energy harvesting and self‐powered system.     

Tailoring Structure-Borne Sound through Bandgap Engineering in Phononic Crystals and Metamaterials: A Comprehensive Review

In solid state physics, a bandgap (BG) refers to a range of energies where no electronic states can exist. This concept was extended to classical waves, spawning the entire fields of photonic and phononic crystals where BGs are frequency (or wavelength) intervals where wave propagation is prohibited. For elastic waves, BGs are found in periodically alternating mechanical properties (i.e., stiffness and density). This gives birth to phononic crystals and later elastic metamaterials that have enabled unprecedented functionalities for a wide range of applications. Planar metamaterials are built for vibration shielding, while a myriad of works focus on integrating phononic crystals in microsystems for filtering, waveguiding, and dynamical strain energy confinement in optomechanical systems. Furthermore, the past decade has witnessed the rise of topological insulators, which leads to the creation of elastodynamic analogs of topological insulators for robust manipulation of mechanical waves. Meanwhile, additive manufacturing has enabled the realization of 3D architected elastic metamaterials, which extends their functionalities. This review aims to comprehensively delineate the rich physical background and the state-of-the art in elastic metamaterials and phononic crystals that possess engineered BGs for different functionalities and applications, and to provide a roadmap for future directions of these manmade materials.     

Alternating Magnetic Field-Enhanced Triboelectric Nanogenerator for Low-Speed Flow Energy Harvesting

Low-speed flow energy, such as breezes and rivers, which are abundant in smart agriculture and smart cities, faces significant challenges in efficient harvesting as an untapped sustainable energy source. This study proposes an alternating magnetic field-enhanced triboelectric nanogenerator (AMF-TENG) for low-speed flow energy harvesting, and demonstrates its feasibility through experimental results. AMF-TENG's minimum cut-in speed is 1 m s⁻¹, thereby greatly expanding its wind energy harvesting range. When the wind speed is 1–5 m s⁻¹, the open-circuit voltage (V_OC) is 20.9–179.3 V. The peak power is 0.68 mW at 5 m s⁻¹. In a durability test of 100 K cycles, the V_OC decreases from 188.4 to 174.2 V but remain at 92.5% of the initial value. furthermore, the AMF-TENG can harvest low-speed flow energy from the natural environment to power temperature and humidity sensors and wireless light intensity sensor in smart agriculture. This study provides a promising method for low-speed flow energy harvesting in distributed applications.     

风能收集型摩擦纳米发电机研究进展

有效地将自然界中的能量转换为电能对于构建环境友好型社会具有重要意义。摩擦纳米发电机(Triboelectric Nanogenerator,TENG)是一种新型的机械能-电能转换装置,可实现将微弱机械能高效地转换为电能。在自然界众多的机械能中,风能因其分布广和储存量大而受到广泛关注。近年来,将风能高效率地转换为电能是TENG技术的研发重点之一。研究人员对此展开了细致的研究工作,获得大量研究进展。一般说来,风能收集型TENG的研究内容主要包括器件结构优化、摩擦起电材料的物理与化学改性以及电源管理电路设计优化。针对这些研究内容,详细介绍了近年来TENG在收集风能方面的研究进展,剖析存在的问题,并对其未来的应用和发展进行了展望。     

Complementary Electromagnetic‐Triboelectric Active Sensor for Detecting Multiple Mechanical Triggering

With the fast development of integrated circuit technology and internet of things, sensors with multifunctional characteristics are desperately needed. This work presents an integrated electromagnetic‐triboelectric active sensor (ETAS) for simultaneous detection of multiple mechanical triggering signals. The good combination of a contact‐separation mode triboelectric nanogenerator (TENG) and an electromagnetic generator (EMG) realizes the complement of their individual advantages. The theoretical calculation and analysis of EMG and TENG are performed to understand the relationship between their output and the external mechanical signals. The experimental results show that the output voltage of TENG part is suitable to detect the magnitude of the external triggering force with a sensitivity of about 2.01 V N^?1. Meanwhile, the output current of EMG part is more appropriate to reflect the triggering velocity and the sensitivity is about 4.3 mA (m s^?1)^?1. Moreover, both the TENG part and the EMG part exhibit good stabilities after more than 20 000 cycles of force loading and unloading. One ETAS that can record the typing behavior of the finger precisely is demonstrated. In addition, the TENG part can harvest the mechanical energy during typing for possible powering of tiny electronics. This ETAS has promising applications in complex human–machine interface, personal identification, and security system.     

Breaking the Tradeoffs between Different Mechanical Properties in Bioinspired Hierarchical Lattice Metamaterials

It is a long-standing challenge to break the tradeoffs between different mechanical property indicators such as the strength versus toughness in the design of lightweight lattice materials. To tackle this challenge, a hierarchical lattice metamaterial with modified face-centered cubic (FCC) cell configuration, inspired by the glass sponge skeletal system, is proposed. The proposed lattice metamaterial simultaneously possesses high strength, high energy absorption, considerable toughness, as well as controllable deformation patterns through integration of both bionic features of double diagonal reinforcement and hierarchical circular modification. The compressive strength and energy absorption can reach 69.13 MPa and 53.39 J cm³, respectively. Furthermore, the proposed lattice also exhibits exceptionally high damage tolerance compared with existing lattice metamaterials with comparable strength by attenuating stress and deformation concentration that may cause catastrophic collapse. This design approach combines the advantages of tensile-dominated and bending-dominated lattices. Quantitatively, in terms of specific strength, specific energy absorption, and crushing force efficiency, the modified hierarchical circular FCC (MHCFCC) lattice metamaterial outperforms the Octet lattice by 14.85%, 53.28%, and 110.52%, respectively. This multibionic feature integration approach provides advanced design strategies for high-performance architected metamaterials with promising application potential.     

More Than Energy Harvesting in Electret Electronics-Moving toward Next-Generation Functional System

Electrets are normally applied for energy conversion from mechanical vibration sources in the environment to electrical power without any friction, which induces electric device sustainability and mechanically robust. It functions for electron storage and electrostatic/triboelectric effect, whose electrical/mechanical performance dramatically benefits energy harvesters, self-powered sensors, and even intelligent/sustainable systems. To summarize the progress of electret-based electronics, this review proposes three key issues around enhanced energy harvesting toward sensors and sustainable systems. First, with the properties of long-term charge storage characteristics and the contactless mechanism for energy harvesting, the enhancement effect in electret from MEMS devices, porous microstructure devices, and multilayer electret devices are carefully assessed with the output power from various devices. Second, the multi-functional applications aspect along with the triboelectric coupling effect and artificial piezoelectric materials are discussed as future electret devices, for example, polydimethylsiloxane materials. Third, more than energy harvesting, machine learning-enabled methodology in electret electronics can be more reliable and sustainable, dramatically contributing to the living standard of the society. Electret technologies on the future development trends are finally analyzed and strengthened toward multifunctional, sustainable, and intelligent systems along with the upcoming technologies in coupling mechanism, artificial composite materials, and machine learning in data fusion.     

A Wrinkled PEDOT:PSS Film Based Stretchable and Transparent Triboelectric Nanogenerator for Wearable Energy Harvesters and Active Motion Sensors

The functionalized conductive polymer is a promising choice for flexible triboelectric nanogenerators (TENGs) for harvesting human motion energy still poses challenges. In this work, a transparent and stretchable wrinkled poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS) electrode based TENG (WP‐TENG) is fabricated. The optimum conductivity and transparency of PEDOT:PSS electrode can reach 0.14 kΩ □⁻¹ and 90%, respectively, with maximum strain of ≈100%. Operating in single‐electrode mode at 2.5 Hz, the WP‐TENG with an area of 6 × 3 cm² produces an open‐circuit voltage of 180 V, short‐circuit current of 22.6 µA, and average power density of 4.06 mW m⁻². It can be worn on the wrist to harvest hand tapping energy and charge the capacitor to 2 V in ≈3.5 min, and then drive an electronic watch. Furthermore, the WP‐TENG as the human motion monitoring sensor could inspect the bending angle of the elbow and joint by analyzing the peak value of voltage and monitor the motion frequency by counting the peak number. The triboelectric mechanism also enables the WP‐TENG to realize high‐performance active tactile sensing. The assembled 3 pixel × 3 pixel tactile sensor array is fabricated for mapping the touch location or recording the shape of object contacted with the sensor array.     

Ultrafast Shape-Reconfigurable Chiral Mechanical Metamaterial based on Prestressed Bistable Shells

Fast shape-reconfiguration with large morphing amplitude is crucial for intelligent materials/structures that require tunable functions and adaptivity to different environments. However, the morphing strategies are rare in combining ultrafast speed, large amplitude, and high energy-efficiency simultaneously. Herein, a class of 2D and 3D chiral mechanical metamaterials are proposed to tackle this challenge based on prestressed bistable metallic shells. The metamaterial is architected by cylindrical cores and slender bistable shells with an anti-chiral arrangement. The bistable shell has a flat extended shape and a rolled-up shape that can wrap on the connected cylindrical cores compatibly, and thus endow the metamaterials with a tunable morphing amplitude that can even extend to infinity. By experiments, simulations and theoretical modelling, it is demonstrated that the bistable shell can transform from the extended state to the rolled-up state with a transitional speed of 7.56 m s⁻¹, which provides the 2D and 3D metamaterials with 25.38- and 101.14-times body area/volume variation per second, respectively. Moreover, a smart trapper for capturing moving objects and a phononic structure with tunable band gaps are realized based on the metamaterials. This work provides a straightforward platform to design metamaterials and their derived systems with ultrafast and large-amplitude shape-reconfigurability.     

Theoretical Model and Outstanding Performance from Constructive Piezoelectric and Triboelectric Mechanism in Electrospun PVDF Fiber Film

Flexible materials with high electromechanical coupling performance are highly demanded for wide applications for electromechanical sensors and transducers, including mechanical energy harvesters. Here, outstanding electromechanical performance is obtained in electrospun‐aligned polyvinylidene fluoride (PVDF) fiber film. A theoretical model is developed from systematic theoretical analyses to clarify the underlying constructive piezoelectric‐triboelectric mechanism in the polarized PVDF fiber films that explains the experimental observations well. The electrospinning process induces polarization alignment and thus tunes the electron affinity for PVDF fibers with different polarization terminals, which results in the constructive piezoelectric and triboelectric responses in the obtained PVDF fiber films. Extremely large effective piezoelectric performance properties are achieved in the direct piezoelectric measurements, reaching the maximum effective piezoelectric strain and voltage coefficients of ?1065 pm V^?1 and ?9178 V mm N^?1, respectively, at 100 Hz. In the converse piezoelectric measurements without a significant contribution from reversible triboelectric effect, the maximum effective piezoelectric strain and voltage coefficients are ?166 pm V^?1 and ?1499 V mm N^?1, respectively. The theoretical analyses and experimental results show the great potential of the electrospun aligned polar PVDF fiber material for various electromechanical device applications, particularly for mechanical energy harvesting.     

Multilayered Cylindrical Triboelectric Nanogenerator to Harvest Kinetic Energy of Tree Branches for Monitoring Environment Condition and Forest Fire

Forest fires present a great threat as they can rapidly grow and become large, resulting in tragic loss of life and property when occurring near occupied land. Here a self‐powered fire alarm system based on a novel multilayered cylindrical triboelectric nanogenerator (MC‐TENG) that can produce electrical power for the detection sensors by harvesting the kinetic energy of moving tree branches in a forest is presented. The major parameters for harvesting the kinetic energy using the proposed MC‐TENG are investigated, including the number of triboelectric layers, the frequency, the amplitude of external excitation, and the orientation between motion direction and device configuration. The fabricated MC‐TENG results in a peak power of 2.9 mW and a maximum average power of 1.2 mW at a low frequency of 1.25 Hz. The integrated self‐powered forest fire alarm system, consisting of fire sensors, a carbon‐based micro‐supercapacitor, and the MC‐TENG, is demonstrated to be able to report fire risk or hazard efficiently, accurately, and robustly. This study provides a new solution to reduce the forest fire risk through a portable and sustainable alarm system by effectively harvesting kinetic energies in natural environment with TENG technology.     

High‐Performance Flexible Thermoelectric Devices Based on All‐Inorganic Hybrid Films for Harvesting Low‐Grade Heat

The fabrication of a flexible thermoelectric (TE) device that contains flexible, all‐inorganic hybrid thin films (p‐type single‐wall carbon nanotubes (SWCNTs)/Sb₂Te₃ and n‐type reduced graphene oxide (RGO)/Bi₂Te₃) is reported. The optimized power factors of the p‐type and n‐type hybrid thin films at ambient temperature are about 55 and 108 µW m^?1 K^?2, respectively. The high performance of these films that are fabricated through the combination of vacuum filtration and annealing can be attributed to their planar orientation and network structure. In addition, a TE device, with 10 couples of legs, shows an output power of 23.6 µW at a temperature gradient of 70 K. A prototype of an integrated photovoltaic‐TE (PV‐TE) device demonstrates the ability to harvest low‐grade “waste” thermal energy from the human body and solar irradiation. The flexible TE and PV‐TE device have great potential in wearable energy harvesting and management.